With the introduction of the 3rd generation (3G) mobile telephony system, the Wideband Code Division Mobile Access (WCDMA) based Universal Mobile Telephony System (UMTS), new opportunities are given to provide even more advanced services to the subscribers. Examples of such services are video telephony, high-bit-rate packet data services etc. The 2nd generation (2G) mobile telephony systems GSM/GPRS have evolved to handle more advanced services. The introduction of EDGE has for example enhanced the packet data services by providing higher bit rates. Many services are supported equally well by both the second and third generation systems such as voice services, SMS, low to medium high bit-rate packet data services etc., while other services such as high-bit-rate packet data services (384 kbit/s) are only supported by 3G. The third generation mobile stations are currently dual mode phones supporting both 2G and 3G in order to cope with the case where one of the network loses coverage.
GSM operators with a UMTS license are interested in still making use of the investments made in the GSM/GPRS/EDGE networks also after building out a parallel WCDMA based network. This will be especially interesting in the future when the number of dual mode mobile station increases and the number of GSM only mobile stations will start to decrease. In the paper “GSM and WCDMA-Common network approach” Ericsson Review, Vol. 81 (2004):2, pp 82-89 by Ros-Marie Furtenback, Torsten Hunte, Dalibor Turina and Ulrik Wahlberg:, a common network approach is presented and an analysis is made of which traffic steering mechanisms are needed in order to achieve a true common GSM/WCDMA network. Such a common GSM/WCDMA network 100 is shown in FIG. 1. The UMTS radio access network (UTRAN) 110 is connected to the common core network 102 via a first controlling entity, the Radio network Controller RNC 106, and the radio network of the GSM network 108 is connected to the common core network 102 via a second controlling entity, the Base Station Controller BSC 104. It should be noted that the BSC is further connected to a plurality of base stations that communicates wirelessly with the mobile stations and the RNC is in a similar way connected to a plurality of base stations, also referred to as Node Bs adapted to communicate wirelessly with the mobile stations. The arrow denoted 112 indicates directed retry or handover. In an ideal common network the subscribers are unaware of which system that provides the services as long as the subscriber perceives a seamless service level in terms of quality of service (QoS). One of the issues that is described in the above mentioned paper is how to distribute the traffic generated by the dual mode mobile stations on the GSM and the UMTS in areas where both systems have a similar coverage. The distribution should ideally be made in such a way that the network resources in both the GSM and UMTS networks are utilized in an optimal way and at the same time providing the subscribers the services they are requesting and in such a way that the subscribers perceive a seamless common network.
It may also be possible that a service is supported in 2G only, e.g. due to a decision of the operator. An MS camping in 3G is then directed to 2G by certain means (e.g. cell re-selection), in order to execute the service. When the service is completed, the 3G camping MS is then returned to the 3G network which results in that the service can not be executed without a further direction to the 2G network. These unnecessary directions between the network causes an increased signalling.
Camping
One of the current approaches is to let the dual mode mobiles camp in the 3G network, i.e. apply the 3G camping strategy. An MS is said to camp on a cell when the MS is in idle mode, packet idle mode and packet transfer mode. An MS camping on a cell has completed cell selection/reselection process and has chosen a cell from which it plans to receive all available services.
3G Camping Strategy
In the 3G camping strategy, the dual mode MSs will camp in the 3G network as soon as the 3G coverage exists regardless of the 2G radio network condition. In case of high load in the 3G network, a part of the service requests, service requests for services supported in 2G typically speech services, are directed to the 2G network either at service establishment (directed retry) or during service (handover of circuit services, cell reselection of packet switched services).
As long as the 2G network has capacity left it accepts the directed retry and handover requests. If the load in the 2G network is high the directed retry and handover requests are rejected. It should be noted that the 2G network is of course also required to handle the 2G single mode mobile stations. Directed retry is a handover during the establishment of a call, i.e. only the signalling channel used during the call establishment is handed over. When the service session ends the dual mode mobiles station returns to idle mode and camps again in the 3G network in order to always have access to the 3G services not provided by the 2G network. Cell-reselection from 3G to 2G may also be performed due to that a requested service is only supported in the 2G network.
One drawback with this alternative is the delayed call set-up times caused by the directed retry and the extra traffic load generated by the handovers and cell reselections. Another drawback with this alternative is the fact that the periodic mobility management signalling (periodic location update) for all dual mode MSs will take place in 3G network and extra mobility management signalling will also take place when the dual mode MS moves between the 2G and the 3G systems. That implies that the number of directed retry and inter system handovers due to traffic load balancing between 3G and 2G network should be kept to a minimum.
Thus, it would be desirable to be able to distribute the dual mode mobile stations in the UMTS and the GSM network. In an ideal common GSM/UMTS network the GSM single mode mobile terminals are camping in GSM together with a portion of the dual mode mobile stations (MS), more specifically the dual mode MSs that do not use “UMTS only provided” services but services that are supported by both GSM and UMTS. The dual mode MSs, that are using services that only are supported by UMTS network, are camping in the UMTS network together with a part of the dual mode MSs using services supported by both GSM and UMTS networks. The distribution of the dual mode MS not using “UMTS only provided” services are distributed in such a way that the UMTS network capacity limit is not exceeded. With the distribution described above the directed retry and handovers between the systems due to traffic load balancing would be minimized. This alternative needs support for directed retry from GSM to UMTS at call set-up, in order to guarantee that subscribers using dual mode MS always have access to all UMTS services even if camping in the GSM system at initiate of the call set-up.
In the above mentioned paper, a solution is disclosed that bases the distribution of idle mode dual mode MSs by restricting location registrations. The restriction can for example be based on the subscriptions related to the dual mode MS. One drawback with this solution is the granularity, i.e. all dual mode MSs with a certain subscription are forced to camp either in GSM or in UMTS. Thus, all dual mode MSs will apply exactly the same algorithm when being in idle mode. With a UMTS camping strategy this implies that all dual mode MSs will camp in UMTS as soon as a UMTS cell fulfils the cell reselection algorithm criteria. Another drawback with this solution is the fact that the load within the radio network is not taken into account when deciding where the dual mode MS shall camp.
The cell reselection criteria from GSM to UMTS are (further details can be found in 3GPP TS 45.008):    1) The quality (measured in Ratio of energy per modulating bit to the noise spectral density) of the UMTS cell is above a minimum quality threshold.    2) The “signal strength” (measured in Received Signal Code Power) of the UMTS cell is above a minimum threshold.    3) The “signal strength” of the UMTS is stronger than the signal strength of the serving and neighbouring GSM cells plus an offset parameter.
A UMTS camping strategy implies that the offset parameter mentioned above must be set to a value promoting the MS to camp on the UMTS cell in the comparison with the GSM cells, otherwise the dual mode MS will be kept in GSM if the GSM cell/s are strong.
With an overlapping GSM and UMTS coverage this means that all dual mode MS will end up in a UMTS cell as soon as a UMTS cell is good enough.
The current telecommunications standards do not provide the network with enough steering mechanisms to control the idle mode camping behaviour of dual mode mobile stations. The cell reselection algorithm performed by dual mode MS take only the radio condition of the GSM and the UMTS network into account. Depending on camping strategy this results in that a majority of the dual mode MSs are camping in the GSM or the UMTS network. The behaviour of the dual mode MSs in terms of which services it uses is not taken into account. This further on makes it impossible to accomplish an optimal distribution of dual mode MS in the common GSM and UMTS network already in idle mode.
To achieve an efficient usage of the resources in the common network at high traffic load, solutions have to rely on existing procedures such as directed retry at call set-up and inter system handover/cell reselection during ongoing calls/packet sessions. However, these procedures imply a delay on the call set-up time, extra traffic is generated in the network nodes and the mobility management signalling increases.
It is also not possible to keep an MS during a service session within the 2G access if mobile controlled mode of operation is used, since the cell reselection strategy in 3G camping, would move the MS back to 3G whenever the algorithm detects that 3G coverage is present and is good enough. This will create delays in switching to 3G and losing data will yield poor quality of service and may also, if the service is only provided in 2G, loose the service session. Certain broadcast services may be sent at different speeds in 2G and 3G and changing access may create a big loss of data.